This invention relates to polishing slurry for texturing the surface of a magnetic hard disk substrate.
With the increase in the memory capacity of magnetic hard disks in recent years, it is becoming required to reduce the floating distance of a magnetic head from the surface of the magnetic hard disk to less than 50 nm. In connection with this requirement to reduce the floating distance of a magnetic head, it is also becoming necessary to increase the density of texturing marks formed on the surface of magnetic hard disk substrates to greater than 80 lines/μm.
In order to reduce the floating distance of a magnetic head, it is generally necessary to prevent both adsorption and collision of the magnetic head to and with the surface of the magnetic hard disk.
In order to prevent the adsorption of a magnetic head, the surface of the magnetic hard disk must have a certain degree of roughness. For this reason, a so-called texturing process is carried out on the surface of magnetic hard disks to form approximately concentric circular indentations and protrusions referred to as texturing marks so as to thereby provide an appropriate degree of surface roughness, as well as to improve the magnetic characteristics by providing a magnetic directionality in the circumferential direction of the surface of the magnetic disk. In order to thus improve the magnetic characteristics, it is necessary to form very fine texturing marks and this means that the pitch of the texturing marks must be made very small or that the texturing marks must be formed at a very high density. For the purpose of preventing collision of a magnetic head, it is necessary that there be no abnormal protrusions on the textured surface of the magnetic hard disks.
In general, aluminum substrates having an alumite processing or a non-magnetic plating process such as Ni—P plating carried out on the surface or glass substrates are widely used as a magnetic hard disk.
A magnetic hard disk substrate is mirror-polished and thereafter subjected to a texturing process for forming texturing marks on the surface. A magnetic hard disk is produced by forming a magnetic layer and a protective layer sequentially on the surface of this magnetic hard disk substrate by a known film-forming technology such as sputtering. Concentric circular protrusions and indentations are formed on the surface of this magnetic hard disk approximately similarly to the texturing marks formed on the surface of the magnetic hard disk substrate.
Since these protrusions and indentations formed on the surface of the magnetic hard disk are the results of the magnetic layer and the protective layer formed on top of the protrusions and indentations on the surface of the magnetic hard disk substrate, they are higher and sloped somewhat more gently than those formed on the surface of the magnetic hard disk substrate. Thus, very fine texturing marks must be formed accurately on the surface of a magnetic hard disk substrate without any abnormal protrusions.
It should be clear from the above explanation that the process of texturing is a very important step in the production of magnetic hard disks (as magnetic memory media) installed in a magnetic recording device or the like which is an external memory device of a computer. Indeed, it is important enough to control the recording density and the reliability of magnetic hard disks.
Such a texturing process is carried out by supplying polishing slurry on the surface of a rotating magnetic hard disk and pressing and running a polishing tape thereon. Approximately concentric circular texturing marks are thereby mechanically formed by means of free abrading particles. In this process, polishing slurry obtained by dispersing particles of one or more kinds of material selected from diamond, alumina and silica in a dispersing medium (dispersant) is used. A tape made of a material such as woven cloth, non-woven cloth, flocked fabric and foamed material is used as the polishing tape.
For forming very fine texturing marks without abnormal protrusions accurately on the surface of a magnetic hard disk substrate, the material, diameter and shape of abrading particles to be contained in polishing slurry have been examined and diamond particles are now coming to be used widely because diamond has superior properties regarding resistance against wear, heat, oxidation and chemicals.
There are two kinds of diamond particles, that is, monocrystalline diamond particles and polycrystalline diamond particles. Monocrystalline diamond particles are polygonal particles with corners around them while polycrystalline diamond particles are rounded particles with no corners around them. Fine texturing marks can be formed on the surface of a magnetic hard disk substrate by a texturing process using such monocrystalline and polycrystalline diamond particles but since scratches and abnormal protrusions tend to result if monocrystalline diamond particles are used, polycrystalline diamond particles are now being used as diamond particles.
As shown in Japanese Patent Publication Tokkai 6-150304 (in paragraphs 0007, 0008, 0017 and 0018), for example, conventional texturing processes using abrading particles comprising diamond particles are comprised of two stages. The texturing process of the first stage is carried out by using a polishing tape having affixed thereon abrading particles with relatively strong grinding power. In the subsequent second stage, abnormal protrusions formed on the surface of the magnetic hard disk substrate in the first stage are removed by a free particle polishing method by using polishing slurry having dispersed therein abrading particles comprising polycrystalline diamond particles with average diameter less than 1 μm. Since polycrystalline diamond particles are round particles with no corners around them, as explained above, having a relatively low grinding power, they can be used conveniently for the purpose of removing abnormal protrusions formed in the first stage. In other words, texturing marks without abnormal protrusions can be formed as a synergistic effect of the first and second stages of texturing process.
This prior art technology is disadvantageous in that two stages are required and that texturing takes time and is costly. Moreover, the second stage is only for removing abnormal protrusions and, since the density of the texturing marks depends only on the first stage, fine texturing marks at recently required high densities (such as 80 lines/μm or higher) cannot be formed.
Japanese Patent Publication Tokkai 11-138424 (in paragraphs 0014-0016) discloses another texturing technology by means of abrading particles comprising polycrystalline diamond particles wherein use is made of slurry obtained by dispersing polycrystalline diamond particles in the form of primary particles within a liquid dispersant. This is because it was believed that the diameters of abrading particles would become irregular if secondary particles (coagulating particles) are present in the polishing slurry, causing scratches and abnormal protrusions to be formed on the surface of the magnetic hard disk substrate and preventing uniform texturing marks from being formed. With such polishing slurry using polycrystalline diamond particles in the form of primary particles as abrading particles, however, texturing marks cannot be formed at densities of 80 lines/μm or higher.
It is generally known that abrading particles with even smaller diameters must be used for forming finer texturing marks and also that abrading particles with uniform diameters must be used in order to form uniform texturing marks. If only the diameters of abrading particles were made smaller in the aforementioned prior art technology as a polishing tape made of woven or non-woven cloth or flocked fabric is used as a polishing tape in a texturing process, the abrading particles would easily pass through the fibers constituting such a polishing tape. If a tape made of a foamed material is used as a polishing tape, such abrading particles would be captured easily inside indentations formed by bubbles on the surface of the polishing tape and would not act uniformly over the surface of the magnetic hard disk substrate. As a result, their grinding force is adversely affected either locally or entirely over the surface of the magnetic hard disk substrate and a surface with uneven roughness would result. In other words, fine texturing marks cannot be formed uniformly and accurately.
Japanese Patent Publication Tokkai 2002-30275 (in paragraphs 0012-0017) discloses another texturing technology using polycrystalline diamond particles as abrading particles wherein use is made of slurry obtained by dispersing in a dispersing medium coagulated polycrystalline diamond particles in the form of secondary particles produced by positively causing polycrystalline diamond particles with diameter of primary particles less than 20 nm. In this technology, the primary particles are caused to coagulate such that apparent diameters of individual abrading particles (secondary particles) become larger (say, 0.05-0.5 μm) and hence the aforementioned problems of the other prior art texturing technologies (such as the abrading particles easily moving through the fibers of a polishing tape) may be overcome.
Moreover, since the plurality of very small primary particles which are around the abrading particles (secondary particles) act on the surface of the magnetic hard disk substrate, fine texturing marks can be thereby formed on the surface of the magnetic hard disk substrate. The secondary particles, which are relatively larger, are broken up by the pressure with which the polishing tape is pressed such that the generation of scratches and abnormal protrusions on the surface of the magnetic hard disk substrate can be inhibited.
If use is made of slurry having coagulated polycrystalline diamond particles (secondary particles) comprising coagulated polycrystalline diamond particles with primary particles having diameters less than 10 nm dispersed in a dispersing medium, the valley portions of the texturing marks become too shallow and there arises the problem that fine texturing marks cannot be accurately created.